To protect against damage to pipelines, corrosion resistant thermoplastic liners have been used in pipelines. Lining of pipelines in such a manner eliminates the need for "digging up" and replacing pipeline which can be very costly.
Heretofore, two types of liners for pipes, loose fitting liners and tight fitting liners, were utilized. A loose fitting liner has been described as one that contains the pressure of the fluid or gas in the liner. A tight fitting liner is flush against and tightly engaged with the interior wall of the pipeline. The tight fitting liner is physically supported by the pipeline and so may depend on the strength of the pipeline to contain the pressure of the fluid or gas in the pipeline. Tight fitting liners offer certain advantages over loose fitting liners in that they may be less expensive, they do not have to bear significant loads, and they maintain the greatest possible inside diameter. Several methods of installing tight fitting liners are described in U.S. Pat. No. 5,072,622 (Roach and Whitehead) and more fully disclosed in Canadian Patent 1,241,262 (Whyman and Kneller), U.S. Pat. No. 3,856,905 (Dawson), U.S. Pat. No. 3,959,424 (Dawson), Canadian Patent 575,424 (Lontz et al), and U.S. Pat. No. 3,462,825 (Pope et al).
The problem with tight fitting liners is that although the liner protects the interior of the pipe from corrosion it does not protect the exterior of the pipe. Corrosion damage may weaken the physical strength of the pipe to the point where it is unable to bear the pressure within and cause leakage. Since the tight fitting liner depends on the strength of the pipeline to contain the pressure of the fluid or gas in the pipeline, if the pipeline fails, the liner will therefore fail. Failure of the liner and pipe will result in significant replacement cost and may be catastrophic to the environment depending on the nature of the substance contained in the pipeline.
Thus, repairing an existing pipeline by installing a liner does not solve all of the problems associated with pipelines. The pipeline must also be monitored continuously for leakage and stress in order that repairs can be made quickly.
Leak detection systems associated with leakage into an annular space between a liner and pipeline are known in the art. These systems only have application in dual-pipe or jacketed pipelines or where a loose fining liner is installed. Systems of this type are not suitable for use in a pipe fitted with a tight-fitting liner because in a tightly lined pipe, there is no annulus between the pipe and the liner.
Another leak detection system involves the use of a so called fluid leak detector cable or sensing cable placed in parallel but external to a pipeline. Such a system is disclosed in U.S. Pat. No. 4,029,889 (Mizuochi) and U.S. Pat. No. 4,095,174 (Ishido). In such a system within the detecting cable there are two electrically conductive elements separated by insulation. A leaking substance will permeate the insulation and cause a change in the electrostatic capacity between the conductive elements, which change can be detected and measured by well-known capacitance bridge circuitry.
In addition to leak detection of pipelines, monitoring and measurement of structural loads and stresses in the pipeline are also important.
The use of optical fiber systems has been employed as a means for locating and measuring stress forces. Short bends in an optical fiber, commonly called microbends, cause an attenuation in the back scattered light intensity from a transmitted light signal and an optical time domain reflectometer (OTDR) has been used to monitor the cause and effects of microbending in optical fibers.
U.S. Pat. No. 4,795,232 (Persson) discloses a fibre optic cable responsive to microbending forces. U.S. Pat. No. 4,42,979 (Asawa) discloses a system for the remote measurement of structural forces by microbending of optical fibers. This system includes an optical fiber extending along the length of the structure in association with a series of microbend transducers. U.S. Pat. No. 4,654,520 (Griffiths) discloses another system for structural monitoring using fibre optics which includes attaching an optical fiber directly to the structure to be monitored.
All of the above mentioned leak detection systems and structural monitoring systems bear the disadvantage of being mounted externally to the pipeline, totally separate and apart from a liner. The tightly fitting liner mentioned above bears a disadvantage of being unable to contain the pressure of the pipe.